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Classification of the Sci
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a heavy body in the air? None at all. Imagine, for example, a horizontal raft through which air is being forced from above to the underside. If this air passes through it as great a rate as the raft would sink in the air by gravity, it will not fall. Now the raft would fall in vacuo at the rate of 16 feet per second. It would therefore be sufficient to pass every second through the raft a cylinder of air having a base equal to the area of the raft and an altitude of 16 feet. But in a hundredth of a second, the raft in vacuo will only fall .0016 feet or about 1/50 of an inch. It will therefore be sufficient if the current of air from above to the underneath carries every hundredth of a second a cylinder of air of the same area of base and an altitude of 1/50 of an inch; that is, in a second it will carry a cylinder of air of the same base and 2 inches instead of 16 feet. What is the explanation of this paradox? It is that we have entirely neglected the dynamics of motion of the air, the effect of which is roughly described as the "resistance" of the air. If it were not for this resistance,